The present invention is directed to a vacuum gauge for detecting and/or measuring the partial pressure of a gas, such as, for example, water vapor or carbon monoxide.
Present vacuum gauges for measuring the total pressure of gases in a vacuum chamber are numerous. They may be classified into three main groups: Mechanical action gauges induced by pressure differentials; thermal conductivity gauges of the residual gases; and ionization gauges of the residual gases.
Gauges of the first group--mechanical action gauges--use a technique that makes the actual pressure of the residual gases perform work that can be measured. Bourdon tubes or mechanical diaphragm gauges are examples. In most cases, this technique is only usable at vacuum levels that do not extend below rough vacuum. One notable difference to this is the capacitance manometer-gauge, where tiny movements of a thin, metal diaphragm are detected with a capacitance-measuring circuit, and can measure up to high vacuum.
Gauges of the second group--thermal conductivity-gauges, are based on a common technique where the rate of heat loss from a heated wire is proportional to the total pressure. The more molecules present in the vacuum, the greater the heat loss. Examples of these gauges are Pirani and thermocouple gauges, and they are used in various degrees, from atmospheric pressure down to 10.sup.-3 torr, or very slightly below.
Gauges of the third group--ionization gauges--use a technique that is the most common for measuring pressures below 10.sup.-3 torr The technique employs a heated cathode that is used as a source of electrons to ionize the residual gases. The ions are collected and the resultant ion current is measured, which measurement is proportional to the pressure.
While the above-described, prior-art vacuum-gauges work well for measuring the total pressure of a vacuum-chamber, they are not used for measuring the partial pressure of a gas in a vacuum-chamber. The need, or desire, to measure and know the partial pressure of a particular gas within a vacuum-chamber is very important for many vacuum processes, where a relatively large presence of the particular gas could adversely affect the vacuum process. For example, in the well-known vacuum processes of sputtering and plasma etching, which processes are performed usually between 10.sup.-3 and 10 torr total pressure, water vapor is ofttimes inadvertantly introduced or liberated from the vacuum-chamber walls while the processes are being carried out. Since water vapor is highly, and adversely, reactive to these processes, it is extremely important to detect and measure the amounts of water vapor present, in order to prevent the loss of the product.
There are many instruments and techniques currently available for detecting and measuring gases and the partial pressure of a gas, where the total pressure is at, or near, atmospheric. For vacuum environments, however, present technology currently offers a mass spectrometer to measure partial pressures of gases. However, the mass spectrometer can only be used at pressures below 10.sup.-4 torr. For higher pressures, the only techniques and instruments currently available to measure partial gas-pressure are complicated, very expensive, gas-handling and pressure reduction systems.
There are many other vacuum processes where the detection of a gas and the measurement of its partial pressure are extremely important. For example, the presence of carbon monoxide in a vacuum chamber has a negative influence and causes a loss of the level of control in the process of crystal growth, where the presence of CO causes surface defects, haze and pits in the crystals. It is extremely important to detect and measure the presence of CO in the vacuum chamber in order to reduce the quantity to acceptable, quality-control levels.